1. Field of the Invention
The present invention relates to an energy trap type vibrating unit which is formed to be capable of preventing or inhibiting vibrational energy of a vibration source from leakage to the exterior.
2. Description of the Background Art
In relation to various apparatuses including vibration sources, generally required are (1) an apparatus which is so formed that vibration from the vibration source will not leak to the exterior, and (2) an apparatus which supports the vibration source without inhibiting its vibrating state.
An example of the apparatus (1) contains a structure which can serve as a vibration source, such as a motor or a compressor, for example. When vibration of such a motor or compressor contained in this apparatus is directly transferred to a chassis or a frame, a fixed portion of the chassis or frame is loosened, or large noise is spread to the surroundings. In such an apparatus containing a vibration source, therefore, vibration of the vibration source must be trapped in the apparatus. Or, it is strongly required that vibration of the vibration source is hardly transferred from the apparatus to another portion such as a floor face in a portion where the apparatus containing the vibration source is installed.
Other examples of the aforementioned apparatus (1) are an analog recorder player and an elevated railway. In an analog record player, it is strongly required that vibration caused by sounds which are generated from a speaker is not transferred to the player. When the speaker serves as a vibration source, therefore, an apparatus which can cut off such vibration is preferably interposed between the speaker and the player, in order to prevent howling. In an elevated railway, on the other hand, vibration is caused following passage of a car, to scatter low-frequency noise to the surroundings. Also in such an elevated railway, it is preferable to prevent the vibration which is caused following passage of the car from transfer to the ground.
On the other hand, an example of the apparatus (2) has a portion which is allowed to positively vibrate at a certain frequency, such as a tuning fork or a piezo-resonator. In such an apparatus, required is a structure for holding the tuning fork or the resonator, serving as a vibration source, without inhibiting the same from vibration. In other words, strongly required is a structure for mechanically holding the aforementioned vibration source without inhibiting the same from vibration.
In the aforementioned apparatus (1), a vibration control structure employing a spring member such as a coil spring or an elastic material such as rubber is generally used as means for inhibiting vibration of the vibration source from transfer to another portion. However, it is impossible to sufficiently cut off the vibration of the vibration source, and hence awaited is a structure which can further effectively prevent transfer of the vibration from the vibration source to another portion.
Also in the apparatus (2), a holding structure using a cushioning material such as a spring is employed in general. However, it is extremely difficult to reliably hold the vibration source such as a resonator or a tuning fork without inhibiting the same from vibration, while the holding structure is complicated.
In general, a piezo-resonator in a kHz band is prepared from (1) a resonator utilizing an expansion vibration mode of a rectangular piezoelectric plate, (2) a resonator utilizing a longitudinal vibration mode for stretching along the longitudinal direction of a bar type piezoelectric member, or (3) a piezoelectric tuning fork resonator.
A piezo-resonator vibrates when a voltage is applied to its resonant part. In order to form such a piezo-resonator as a practical component, therefore, it is necessary to support the piezo-resonator not to inhibit the resonant part from resonance. An energy trap type piezo-resonator can be mechanically supported in a region other than its resonant part, since vibrational energy is trapped in the resonant part. Considering application to a product, therefore, such an energy trap type piezo-resonator is easier to utilize. Thus, the energy-trap type piezo-resonator is desirable for a piezo-resonator in a kHz band.
In the aforementioned well-known resonator (1) utilizing an expansion vibration mode or the resonator (2) utilizing a longitudinal vibration mode, however, it is extremely difficult to trap vibrational energy. In a piezo-resonator 201 utilizing a longitudinal vibration mode shown in FIG. 1A, therefore, spring terminals 202 and 203 are adapted to hold a node of vibration, thereby holding the piezo-resonator 201. Also in a rectangular plate type piezo-resonator utilizing an expansion vibration mode, which is incapable of trapping vibrational energy, spring terminals are adapted to hold a node of the resonator. Thus, it is extremely difficult to form a piezo-resonator utilizing an expansion vibration mode or a longitudinal vibration mode in a kHz band as a surface-mountable miniature chip-type component, due to complicated structures of the elements.
In a piezoelectric tuning fork resonator 206 comprising a piezoelectric plate 204 which is polarized along the direction of thickness, slits 204a to 204c which are formed in the piezoelectric plate 204 and vibrating electrodes 205a (that provided on the rear surface is not shown) which are formed on both major surfaces around the central slit 204b as shown in FIG. 1B, on the other hand, energy is trapped in a vibrating portion. Therefore, this resonator 206 can be formed as a surface-mountable chip-type component since no fluctuation is caused in its characteristics even if the same is held in portions close to edges 204d and 204e of the piezoelectric plate 204.
However, the piezoelectric tuning fork resonator 206 can merely ensure a bandwidth in a range of only about 2% of the resonance frequency due to restriction in mode, although the same can trap energy. Thus, the piezoelectric tuning fork resonator 206 cannot satisfy strong requirement for a piezoelectric resonator ranging over a wide kHz band, which is awaited in the market.